Thrombotic thrombocytopenic purpura (UP) is a thrombotic microangiopathic disorder that is associated with a deficiency of a protease activity that depolymerizes von Willebrand factor in plasma. Familial TIP is caused by a genetic deficiency of this protease, whereas acquired UP is caused by the presence of autoantibodies to the protease. This protease, which has been named vWF cleaving protease (VWFCP), is a metalloprotease and specifically cleaves the Tyrl 605-MetI 606 bond in the P2 domain of the vWF subunit. VWFCP has been purified to homogeneity, partially sequenced, and its cDNA has been cloned. VWFCP is a member of the ADAMTS family of metalloproteases and contains structural motifs such as disintegrin, Cys-rich, and thrombospondin-1 motifs, which are the defining characteristics of this family. In this application, we propose to (1) study the structure and function relationship of VWFCP, and compare the biochemical properties of VWFCP to its naturally occurring variant forms, which are synthesized as a result of alternative mRNA splicing; (2) develop alternative substrates, improved assays for the quantitative measurement of VWFCP activity, the level of VWFCP antigen in plasma, and the titer of autoantibodies to VWFCP. These studies would provide a better understanding of how this metalloprotease regulates the function of vWF and keeps a delicate balance between thrombosis and primary hemostasis. This understanding would provide a basis for devising alternative methods for treating familial and acquired UP.